1. Field of the Invention
The invention relates to an inductive power coupling device for coupling electrical power between two units that are rotatable against each other, specifically for power couplers used in computer tomography scanners. Such power couplers are also known as rotary joints.
2. Description of Relevant Art
In computer tomography (CT) scanners and other related machines high-power in the range from 10 kW up to more than 100 kW is transferred from a stationary side to a rotating side. There, a high voltage in the range of above hundred kilovolts is generated to produce x-ray radiation.
In U.S. Pat. No. 7,054,411 a multiple channel inductive rotary joint is disclosed. It has inductive channels for transferring power from the stationary side to the rotating side. There is an auxiliary power and a main power circuit. Furthermore a capacitive feedback link for power control is provided.
A contactless rotary joint with safety function is disclosed in EP 2 530 805 A1. The inverter of an inductively coupled rotary joint has two operating states. In a first operating state, it receives a three phase power line input. In a second operating state, it receives a single line power input. Depending on the input signal, a higher output voltage and a lower output voltage are generated at the secondary side, which may be used to distinguish between different operating states. The disadvantage is that high power contactors are required for switching over the input signal.
A general problem exists in all mentioned inductively-coupled rotary joints when switching the output power on. At the secondary side of the rotating transformer there are a rectifier and a filter capacitor. When the secondary side is switched off, the filter capacitor is discharged. For switching the secondary side on, the filter capacitor must be charged to the nominal output voltage. Without any current limiting means, there would be a very high current when starting the circuit, until the filter capacitor is charged. This may lead to a significant stress or even overload of associated electronic components.